Intermittent fasting is one of the most wide-sweeping health trends to emerge in recent years, with celebrities far and wide touting the benefits of their time-restricted eating regimens. From Gisele Bündchen to Kourtney Kardashian, Vanessa Hudgens, Jen-An (swoon), and Halle Berry (double swoon), it's hard to move for the amount of good press its been reaping. But, what exactly is intermittent fasting? Is fasting for weight loss even a safe option? And, how could it work for you?
Well, you're in the right place, because we've called upon the best in the biz to explain and advise exactly how to get it right – and, potentially, when to back off. (There is such a thing as too much of a good thing, y'know.)
Before we jump into it, we do need to say that fasting is categorically not for everyone. If you have a history of, or an active eating disorder, are pregnant, underweight or find periods of deliberate fasting to be triggering, please do not undertake any sort of fasting regimen. As always, please consult a GP before undertaking any fasting protocol.
What is intermittent fasting?
Let's begin with the basics: Intermittent fasting is an umbrella term for restricting your daily eating to a specific and predetermined time-window.
There are multiple protocols that fall under this term, with the 5:2 diet being one of the most popular, however, the celeb-fav 16:8 diet also falls into this category – this is when you would fast for 16 hours, taking in only water, and eat in an eight-hour window. Other common regimens include Circadian rhythm fasting – fasting for 13 hours, eating for 11 hours.
Depending on which protocol you choose (more on this later), you'll be fasting for a set period of time and eating only during a certain window.
The reported benefits are far-ranging, from weight loss to better sleep, improved brain function, and a happier digestive system. Colour us intrigued...
How does intermittent fasting work?
Since the dawn of time, humans have been governed by something called Circadian rhythms. These rhythms are internal processes based on day and night – keeping us in line with the natural world around us and maintaining our sleep and wake cycles, internal temperature, and digestion. Cheers, nature!
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However, these rhythms also determine which chemicals are secreted when, and, according to research, can be impacted by eating out of sync with our natural rhythms. Fasting could help get your body back to equilibrium.
It also can help maintain a calorie-intake that fits with your goals. Whether you're trying to lose fat or maintain your healthy weight loss, shortening the amount of time you have to eat can be an easier way to keep your intake in check.
6 benefits of intermittent fasting 1. Stabilizes blood sugar levels
A University of Illinois at Chicago study displayed that eating on a time-restricted regimen can report a reduction in blood sugar levels and insulin resistance – key markers that control how your body breaks down food into energy, uses it for activity or stores it as fat.
2. Improves memory, brain function, and mood
Not to get too technical, but there's a specific protein – brain-derived neurotrophic factor (BDNF) that is elevated during prolonged periods of fasting.
'This protein interacts with the part of the brain that strengthens memory and learning, improving a person's cognitive ability,' explains consultant psychologist and co-founder of My Online Therapy, Dr. Elena Touroni.
According to Dr. Touroni, it can also have a profound impact on your mood, as fasting increases the levels of certain chemicals – like dopamine [the pleasure hormone] which can increase happiness and confidence whilst reducing anxiety.
3. Reduces inflammation
Chronic inflammation can play havoc with your body's immune system and some research has shown that sustained inflammation can lead to chronic health conditions, including heart disease and cancer. This University of Hail study found that intermittent fasting for a month had a noticeable impact in suppressing inflammatory cells, decreasing body fat, and circulating disease-fighting white blood cells.
4. Can help with fat loss
If fat loss is one of your health goals, time-restricted fasting can be a simple way to help stay within your daily recommended calorie amount.
'In terms of fat loss, intermittent fasting works the same way as any other diet – it implements a calorie deficit,' says Maximuscle trainer, Dan Lambert. 'Intermittent fasting simply makes "eating less" easier as you have less time to do it.'
However, and this is important to note, it's not necessarily the fasting for weight loss that's helping you see results.
'It's unclear whether you would lose more fat using a fasting protocol, compared with more conventional eating patterns, where you're eating the same amount of calories,' says Lambert. 'Intermittent fasting may simply allow you to sustain a calorie deficit with relative ease for a longer period of time.'
5. Muscle preservation
'Contrary to popular belief, your body will not start to eat itself if you miss a meal,' explains Lambert. 'Weight loss studies have actually found that when comparing people [those using fasting protocols and those not], intermittent fasting participants actually preserved more muscle tissue during calorie restriction than their unrestricted counterparts.'
6. Simple to follow
Easy to slot into already busy schedules and a simple way to control your daily intake, remembering to eat between certain hours can be a more straightforward way to diet, than food-combining or calorie counting, for example.
Is intermittent fasting a good way to lose weight?
One of the most pressing questions people have when first learning about fasting for weight loss, is if it's a "good" (read: safe and sustainable) way to lose weight.
According to Ro Huntriss, clinical-lead dietician and nutritionist for EXALT, 'intermittent fasting can be a sustainable way to lose weight if it works for you and you're able to go for longer periods of time without feeling hungry.'
By restricting eating to a certain window, you're less likely to pick up the extra calories you might take in by 'grazing' throughout the whole day.
However, Dr. Alka Patel suggests looking at your current lifestyle to find the culprits of any unwanted weight gain before going HAM on a new fasting regime.
'Consider your own pathway to weight gain: sleep and stress affect weight through the cortisol effect: if these are your predominant pathways to weight gain then these need to be addressed first before insulin and fasting,' she says.
Put simply, get your house in order before tweaking your eating schedule. Not to rinse the metaphor, but without a good foundation the whole structure becomes wobbly and we wouldn't want that, would we?
How long should you fast for?
How long you decide to fast for will depend on your own preferences and schedule. Someone working shifts might have very different eating patterns to someone working from home to someone in the office, for example. Here are some of the most common fasting regimens:
'The 16:8 diet is where you fast for 16 hours (including the time your sleep) and you eat within an 8-hour window. So, fast for 16 hours and eat across 8 hours of each day. Some people may also adapt the duration of this window,' explains Huntriss.
The 5:2 diet, however, is slightly different.
'Significantly reduce your calorie intake on two days of the week, to approximately 500-600 calories per day,' explains Huntriss. The other days you would eat your regular calorie amount, making sure not to restrict, as you've already created a calorie deficit by fasting two days per week.
'The fasting days can be changed each week, as long as there are two fasting days in every seven,' she finishes.
Other protocols include 14: 10 and 13:11. Longer fasts can include up to 20+ hours.
How to get started with intermittent fasting
Ready to dive in? OK. Dr. Patel has some guidelines on how to get started.
'To make intermittent fasting a sustainable part of your lifestyle start slow and create regularity through persistence,' she suggests. 'Start with a 12-hour eating window for one to two weeks, then change what you eat by deprocessing your food and boosting fiber in your food choices; once this is established, reduce your eating window to 10 hours or 8 hours every day.'
Basically, don't go too hard, too fast, and quit because it all feels overwhelming. Slow, steady, incremental change is the key to establishing long-term results.
What to eat when your fast finishes
This might seem counterintuitive to a protocol centered on fasting, what you're eating during the non-fasting period is equally as important, too.
'If you consume lots of high-calorie or processed foods during the time in which you do eat, the positive effects of fasting can be lost,' explains Huntriss. 'It's crucially important to focus on what you are eating to make sure you nourish your body adequately to prevent any nutritional inadequacies in your diet and reduce the risk of health issues.'
Huntriss recommends focusing on whole, nutrient-dense foods, such as:
To work out how many calories your body needs – and from which foods specifically, peep our handy macro guide. It'll help calculate your BMR (Basal Metabolic Rate) – how much food your body needs to exist and then show you how to tweak that number depending on your activity levels and goals.
Once you know how much to eat, you can mold your daily food around whichever intermittent fasting routine works best for your schedule.
Psssst! Peep this for the full breakdown of how to calculate your macros.
How fasting can affect your exercise routine
According to Third Space PT and osteopath Henry Howe, Head of Movement at Magic Mountain, it can take a little while to get used to exercising whilst intermittent fasting. One thing to consider, he stresses, is workout timing. 'You may usually be a morning person when it comes to your training, but if you know your fast ends at, say, 12pm – it can be a good idea to time your workout so you can refuel immediately afterward.' Post-workout noms? Count us in.
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'In terms of your actual workout, not much will need to change,' Howe says. However, 'if you're into lifting weights, it's likely your overall volume (weekly sessions) may need to come down slightly, whereas if you're more into cardio, make sure you keep an eye on your hydration.'
Pretty much, listen to your body. It'll tell you if it likes working out without fuel – if you feel intense hanger, irritability, dizzy, or overly fatigued, you might be better chowing down on something before hitting the gym.
4 signs intermittent fasting isn't right for you
So, you've tried it and now you're wondering if it's actually the right fit. We get that, not every health trend will work for everyone. However, there are some clear and common signs that your body is just not getting on with it. Much like those ill-fitting Spanx or that flakey friend, we're here to set you straight on when you should cut and run.
According to Dr. Touroni, there are some common symptoms to look out for:
1. You're feeling noticeably more anxious or stressed
'Although fasting can increase levels of dopamine [a neurotransmitter that helps you feel and enjoy pleasure], it can also increase levels of cortisol [the stress hormone] which can make you feel incredibly anxious and stressed,' she explains. If you're feeling noticeably more stressed and anxious for a sustained period of time, this might be a sign intermittent fasting might not be the fit for you.
2. You're experiencing constant mood swings
Whilst it's fairly normal to feel more irritability in the initial stages of fasting, Dr Touroni says, constant mood swings are a sign that something isn't working quite right.
If scoffing a banana pre-workout keeps hanger at bay (and also makes you feel like a happier, more stable person), then that might be a clear sign your body prefers consistent eating patterns as opposed to extended fasting and feeding windows.
3. You're always tired
'Fasting can decrease the amount of REM [rapid eye movement] sleep, due to the rise in cortisol and insulin whilst fasting,' Dr. Touroni explains.
REM sleep is credited with improving your memory, mood, and learning – so skimping on it isn't the smartest choice.
Also, if you haven't got a handle on what you're eating when you're not fasting, you might be missing out on crucial vitamins, minerals, and macronutrients that keep you happy and healthy.
4. Your menstrual cycle changes
Changes to your menstrual cycle can be a clear sign that something's happening in your body that might not be so great. If you find you